The invention relates to a cyclic process for the preparation of cyclohexanone oxime from cyclohexanone and a hydroxyl-ammonium salt which includes the steps of preparing a solution of the required hydroxyl-ammonium salt and of reacting the hydroxyl-ammonium salt with cyclohexanone.
British Patent Specification No. 1,283,894 discloses a process wherein a buffered aqueous reaction medium showing an acid reaction is recycled between (1) a zone for the synthesis of hydroxyl-ammonium ions by means of molecular hydrogen, and (2) a zone for the synthesis of cyclohexanone oxime, in which the resulting hydroxyl ammonium ions react with cyclohexanone to form cyclohexanone oxime. In this process the buffering action of the reaction medium is due to the presence of buffer acids and salts thereof, in the reaction medium, e.g., phosphoric acid and/or bisulphate as a buffer acid with a phosphate and/or sulphate, respectively, as a buffer salt.
Before the recycled aqueous reaction medium is passed into the zone for the synthesis of hydroxyl-ammonium salt, it is enriched with the required nitrate ions either by addition of nitric acid or by absorption of nitrous gases in the aqueous reaction medium, in which instance nitric acid is formed in situ.
The catalyst used for the reduction of the nitrate ions is, for instance palladium or a palladium-platinum alloy, and the catalyst support is, for instance, carbon or aluminum oxide. The carrier is loaded with catalyst so that the supporting material contains, e.g., 5-20% by weight of catalyst.
The aforementioned reference discloses that the activity of such a catalyst is adversely affected if the catalyst is contaminated with organic substances, such as the cyclohexanone to be converted or the cyclohexanone oxime product. To overcome this problem, the reference teaches reducing the total content of dissolved ketone and oxime in the recycling liquid to a value of at most 0.02% by weight by heating the recycling liquid at a temperature of at least 50.degree. C in the presence of nitrous gases, before the recycling liquid is passed into the zone for the synthesis of the hydroxyl-ammonium salt. The required amount of nitrous gases can then be adjusted by adding nitrous gases or an alkali nitrite to the recycling liquid.
This known process for lowering the ketone and oxime content has the following drawbacks:
1. A large amount of recycling liquid per ton of final product must be subjected to said heat-treatment.
2. If an ammonium salt is used as a buffer salt in the recycling liquid, the ammonium-ion content may become too low, because the known reaction of ammonium ions with nitrous gases with simultaneous formation of nitrogen at a temperature of over about 40.degree. C appears to proceed at least equally as quickly as the reaction in which the catalyst poisons are rendered harmless.
3. As any hydroxyl-ammonium salt present during the heat-treatment may decompose, the overall process efficiency in using an excess amount of hydroxyl-ammonium salt in order to obtain cyclohexanone oxime with the lowest possible cyclohexanone content is not always controllable.